So tried to access mailbox using Outlook, but it errors stating ‘Server cannot be found’. I can access mailbox from OWA (open another mailbox option) but when try to open mailbox as an additional mailbox in Outlook it fails.

Checked for Mapi protocol and found it to be enabled in mailbox features. Tried moving mailbox to another database, disconnected mailbox from original AD account, created new user account and reconnected to new user account to check for AD permission issue, still can’t access mailbox.

Solution:

Try to run New-MailboxExportRequest command for other users and if it works then the issue might be to a particular user mailbox. Please run following command to verify the issues related to problematic mailbox:

Get-Mailbox user1 | FL

Get-MailboxStatistics user1 | FL

Also check for AD Users and Computers and verify the attribute values for: LegacyExchangeDN and HomeMDB. LegacyExchangeDN value for any Exchange recipients (mailbox, resource mailbox, linked mailbox, contact, groups etc.) should be unique across AD forest.

While SMTP addressing is the de facto e-mail addressing standard, Exchange internally still uses an X.500 addressing scheme. Using X.500 implies that an X.500 is required, which is why mail objects in an Exchange organization such as mailboxes, require a properly populated legacyExchangeDN.

Clients like Outlook cache information like the legacyExchangeDN for name lookups.